The present invention relates to an apparatus and methods for feeding fluidized gas into a fluidized bed reactor. The invention is especially suitable for pressurized fluidized bed systems and for feeding fluidizing gas into a reactor to start up the fluidization of the bed.
The invention relates more particularly to a distributor or grid plate in the fluidized bed reactor, disposed between the bottom part of the reactor chamber and wind box beneath the reactor chamber, the plate having nozzles for delivering fluidizing gas into the reactor chamber from the wind box.
Fluidized bed reactors are used, e.g., in combustion, gasification and steam generation processes. Typically, fluidized beds include a lower grid plate for supporting the bed of particulate material which usually includes a mixture of fuel material, inert bed material and eventual adsorbents or catalysts for SO.sub.x or NO.sub.x removal. The grid plate typically has a plurality of gas or air passages therethrough. A wind box or air chamber is disposed below the grid plate and gas or air is introduced into the wind box under pressure. A fan or blower is conventionally used for delivering pressurized gas into the wind box. The gas flows upwardly through the gas passages in the grid plate and suspends the bed above the grid plate in a fluidized state. As a result of the fluidized state, good mixing of gases and solids in the reactor chamber is achieved. This provides relatively uniform temperatures in the reactor, effective combustion processes, good heat transfer characteristics and favorable adsorption of SO.sub.x and NO.sub.x.
Trouble-free operation of the bed requires a certain fluidizing velocity of the gas or air supplied the reactor chamber and an even distribution of gas is essential for processes taking place in fluidized beds. The gas blower must also overcome the flow resistance of the nozzles or the pressure drop (dp) over the nozzles to give a predetermined minimum gas flow which results in a desired fluidizing gas velocity above the grid. A minimum pressure drop dp.sub.min over the grid is required as well to prevent the bed material from flowing back through the nozzles into the wind box. Furthermore, if the pressure drop is too small, the distribution of gas through the nozzles tends to become uneven, the gas flowing mainly through some of the nozzles and not at all through others. Accordingly, nozzles with a suitable minimum pressure drop also at low load conditions should be chosen for the grid plate. The pressure drop over the grid, on the other hand, must not rise too high as this would result in unacceptably high power demands on the gas blower. A blower that could overcome very high pressure drops at high load conditions, does not, on the other hand, function very well at low load conditions. With the technique of the present invention, the holes or nozzles are designed to allow a sufficient amount of gas to flow through the grid at normal reactor pressure, temperature and load.
A pressure drop of about 1-10 kPa over the grid is usually acceptable.
The pressure drop in a nozzle depends on
the construction of the nozzle, (k)
the flow through the nozzle, e.g., the flow velocity (v)
the density of the fluidizing gas (.sigma.) according to the following formula: EQU dp=k*.sigma.* v.sup.2 /2 (1)
The pressure drop (dp) in a nozzle is increased when (a) the capacity of the reactor is increased, that is, when the gas flow (v) is increased or when (b) the density (.sigma.) of the gas is increased, e.g., by increasing the pressure in the reactor.
If, for instance, the gas flow is increased five- to tenfold, the pressure drop will increase twentyfive- to hundredfold. If, at the same time, the pressure in the reactor is increased about tenfold, the pressure drop will grow 250- to a thousandfold. This, of course, is not acceptable.
The pressure drop can be decreased by using mechanically controlled nozzles in which the cross-section of the gas passages can be changed, increased or decreased, to keep the pressure drop at a permitted level. But mechanically controlled nozzles are very delicate devices and do not operate satisfactorily under the operating conditions in a fluidized bed. The nozzles have to be made very accurate and, accordingly, necessarily are formed of expensive materials. They would hardly function in furnaces at high temperature conditions.
Other methods for changing the pressure drop over the grid have been suggested. In U.S. Pat. No. 4,429,471, there is suggested the use of a nozzle having a nipple, a cap and a bushing, wherein the cap is provided with at least one gas distribution passage. The pressure drop in the nozzle is changed by removing the bushing from under the grid plate and threading in another bushing with the required orifice size. This, of course, cannot easily be done during operation of the reactor.
In U.S. Pat. No. 4,648,330, there is suggested a complicated arrangement of nozzles, connected to separate fluidizing gas delivery means, and situated in successive ranged planes. The highest nozzles are initially supplied with fluidization gas at the start-up of the supply of fluidization gas.
The start-up of the fluidized bed reactor is always a problem, especially the start-up of large size units. It has been suggested to start up the reactor by fluidizing first one part of the reactor and then gradually the whole cross-section of the reactor.
In U.S. Pat. No. 4,453,494, there is suggested a start-up method for a fluidized bed reactor, where the bed is made up of a plurality of modules. The fluidizing air system is also compartmented, so that the air flow to each module can be independently controlled. Two start-up burners are located in two modules, to establish the initial combustion. These modules do not contain heat exchange surfaces and can be rapidly brought up to ignition temperature. After these two modules are ignited, coal and air are supplied to the adjacent sections so that the combustion of the coal can be progressively spread from one module to the other. In this method, however, the bed material tends to build up on non-fluidized modules and thus destroy the parameters under which the bed operates. This can at least partly be overcome as is shown in U.S. Pat. No. 4,436,507 by using air nozzles to dissipate the accumulated bed material. As a result, the height of the bed can be maintained substantially level.